1. Field of the Invention
The present invention relates to blood treatment apparatus including a circuit for circulating blood outside the body and designed to be connected to a patient via a single needle or "cannula", this apparatus being, for example, an artificial kidney, a plasmapheresis apparatus or a blood oxygenation apparatus.
2. Description of the Related Art
With such apparatuses, it is conventional for the circuit for circulating blood outside the body to be connected to the patient either by means of two needles with blood being withdrawn from the patient and returned to the patient simultaneously via respective ones of the needles, or else via a single needle with blood then being withdrawn and returned alternatively via the single needle.
The advantage of making a connection by means of a single needle is that it halves the number of punctures that need to be made through the skin of the patient, and with chronic treatments such as dialysis for kidney failure, this may be of crucial importance since access points to the blood in a human body are limited in number and need to be used sparingly.
However, the way in which a single-needle blood treatment apparatus operates suffers from drawbacks because of the need during each withdrawal and return cycle to store a volume of blood in the circuit outside the body, with the stored volume being at least as great as the volume actually treated during the cycle. This need does not arise when the circuit for circulating blood outside the body is connected to the patient via two needles, since blood flows continuously therethrough.
As a matter of fact, in present circuits adapted for operation with a single needle, this volume of blood is generally stored both before and after treatment (in some circuits, after treatment only) in two expansion chambers disposed respectively upstream and downstream from the blood treatment device proper (hemodialyzer, oxygenator, etc.), with the apparatus as a whole giving rise, in operation, to different and varying pressures in said chambers, thereby giving rise to varying pressure differences and, where applicable, to varying flow rates through the treatment device.
Unfortunately, depending on the type of treatment device such pressure variations may be most undesirable. This is particularly true of hemodialyzers in which pressure variations run the risk of rupturing the membrane, may give rise to undesired retrofiltration or ultrafiltration, and make it difficult or even impossible to control an ultrafiltration flow rate accurately when ultrafiltration is desired. As to the variations in blood flow rate through a hemodialyzer, they give rise to reduced performance, and in particular to a loss of renal clearance.